1. Field of the Invention
The present invention relates to a wire bonding apparatus, and more particularly, to an ultrasonic horn supporting structure.
2. Prior Art
The ultrasonic horn supporting structure in a conventional wire bonding apparatus is shown in FIG. 3.
In this structure, a capillary 2 through which a wire (not shown) passes is mounted to one end of the ultrasonic horn 1, and an ultrasonic vibrator 3 is fastened to another end thereof. A flange 4 is formed on the ultrasonic horn 1 so as to positionally correspond to a node of the ultrasonic vibration, and this flange 4 is connected to a cylindrical horn supporter 5. The horn supporter 5 is fastened to a bonding arm 6, and the bonding arm 6 is fastened to a supporting shaft 7. The supporting shaft 7 is rotatably supported on a bonding head (not shown), either directly or via a lifter arm.
Examples of wire bonding apparatuses of this type are described in Japanese Patent Application Laid-Open (Kokai) Nos. H5-34734, H6-196532 and H10-303240.
Generally, as the speed of the operation of a wire bonding apparatus increases, the mechanical vibration of the ultrasonic horn also increases. In particular, vibration of the ultrasonic horn during vertical movement is a problem.
In the prior art, the ultrasonic horn is supported at one place that corresponds to a node of the ultrasonic vibration. Accordingly, the strength of the horn supporter is insufficient; and vibration of the ultrasonic horn generated by an improved speed increase of bonding apparatuses cannot be suppressed. When such vibration occurs, an excessive force applies during bonding to the ball formed at the end of the wire, resulting in an unsatisfactory shape in the crushed ball. Especially for balls having diameters reduced as a result of the use of finer pitches in semiconductor devices, this problem is difficult to deal with.
Furthermore, the flange 4 in the above prior art is integral to the ultrasonic horn 1. Since the frequency of the ultrasonic horn 1 differs from horn to horn for structural reasons, it is difficult to achieve strict alignment of the mechanically worked flange 4 with the frequency node. When a discrepancy occurs between the node position and the flange 4, the flange 4 vibrates so that a loss of vibrational energy, i.e., a so-called "leak", occurs, so that energy unrelated to the bonding conditions is consumed. As a result, the impedance increases, and oscillation that deviates from the optimal frequency occurs, thus causing a deterioration in the bonding quality. Furthermore, since unnecessary power must be injected, unnecessary ultrasonic energy continues to be applied after the completion of bonding, thus causing, for instance, an increase in unsatisfactory crushed ball shapes, stripping of balls and damage to the substrate.